FFSK demodulators are known from, for example, European Patent Application No. 0396970 assigned to Storno A/S. Such known demodulators use 64 times up sampling before low pass filtering the signal to provide the data bit stream. When such a technique is used, the filter to remove the unwanted signals can be made as a simple first order low pass filter. The drawback of this method is a complex and gate consuming up sampling network, which also has high current drain.
The company Consumer Micro Circuits Ltd supplies a decoder circuit having the product designation FX419 which uses a combination of a digital and an analog filter to provide the appropriate attenuation of unwanted signals. In more detail, the circuit provides a receive band pass filter which filters the received FFSK signal, followed by a limiter, followed by a monostable "one-shot" which doubles the frequency of the signal, coupled to and followed by a digital filter which in turn provides its output to an analog low pass filter which provides the demodulated bit stream, which is limited in a limiter to provide an unclocked squared receive data stream. The use of an analog filter is disadvantageous, as it is unsuitable for implementation in a gate array for other digital implementation and is therefore difficult to integrate and miniaturise.
There is a need for an improved FFSK demodulator.